The most prevalent form of a mobile wireless communication system is a cellular network. In such a network, a territory serviced by it is divided into a plurality of geographically substantially distinct, but normally overlapping cells. Within each cell is a base station at which there is an antenna or antenna array connected to a bank of radio transmitters and receivers (hereinafter "radios") for communicating with mobile radios (phones) within the territory. These base stations are sometimes controlled by a base station controller (BSC). The base stations are connected through the base station controller by data and voice links to a mobile telecommunications switching office (MTSO) or mobile switching center (MSC). The MSC connects calls between two mobile radios within the network, between the mobile radios and the public switching telephone network ("PSTN") and occasionally between a mobile radio of that system and a radio of a foreign mobile system.
Each cell has assigned to it specific frequencies or channels on which mobile radios can operate. The channels are not the same as those assigned to immediately adjacent cells to prevent interference. Within each cell, there are normally at least two channels, called control channels, used to transmit data between mobile radios and the base station. This data is used for several purposes, including use by the MSC to signal a specific mobile radio as to which channels are to be used by that specific mobile radio for transmitting and receiving a specific call. As the specific mobile radio nears the edge of a cell, the weaker signal strength is noted by the base station and by the MSC, and arrangements are made, using prior art standardized techniques, to determine to which, of the one or more adjacent cells, the mobile radio is likely to pass. When the mobile radio passes into an adjacent cell, the MSC performs a "handoff" operation in which the MSC instructs the mobile radio to switch to new channels for communication with the base station of the next cell and, simultaneously, arranges for connecting the call through to the base station of the new cell and transmission on the new channels. Arrangements are also made with the base station of the cell that was just left by that radio to release the channels that were being using by the radio that was "handed off".
In a cellular system completing handoffs is critical. Generally, subscribers are very intolerant of dropped calls. To avoid dropping a call as a subscriber moves from one cell to the next, a cellular network must ensure that channels are always available for continuing the call. If there are not any available channels, the call is "blocked." Operators of cellular systems, therefore, effectively give higher priority to handoff calls than to new calls by reserving a certain number, of the total number of channels available to that cell, specifically for handling handoff calls. These reserved channels are typically designated in the industry as "guard" channels.
The grade of service for a trunk line or a cell has been estimated for many years using a well known "Erlang B" model and equation typically found in the form of a table. In other words the likelihood that a call, offered to a group of circuits, will fail to find an idle circuit on the first attempt (will be blocked). An example blocking probability might be 1.1% However, the Erlang B model or equation was developed for wireline service and does not take into account the mobility (handoffs/call) of the mobile radios within a cell. Thus, the Erlang B formula overestimates the traffic capacity of cellular networks by anywhere from 5% to 35% depending upon the number of channels in a cell and the mobility of subscribers to and from that cell. Consequently, the Erlang B formula cannot accurately or efficiently be used to determine the probability for handoff call blocking and thus, used to determine whether a given quantity of guard channels is appropriate. Prior art attempts, to use the Erlang B formula, have consistently overestimated the number of handoff guard channels required and thereby unduly limited the number of channels available for new calls in a given cell.
Unfortunately, the number of mobile phones, terminals or radios within any given cell and the call traffic can, in some cases, fluctuate dramatically during the course of the day. These fluctuations, if rapid, cannot be easily predicted or anticipated. This problem is exacerbated as cell sizes become smaller to accommodate increased numbers of subscribers in a communication area or network. It will be readily apparent to anyone skilled in the art that smaller cells will require more handoffs. Also, any changes in mobility patterns and subscriber mobility will more drastically affect the blocking probability of any calls, whether new or handoff, when cell sizes become smaller.
One prior art article attacks the mobility problem using priority schemes. The article is entitled "Traffic Model and Performance Analysis for Cellular Mobile Radio Telephone Systems with Prioritized and Non prioritized Handoff Procedures" and authored by D. Hong and S. S. Rappaport in Transactions on Vehicular Technology V-35, No. 3, published August 1986 page 77-92. The problem of changing traffic loads has also been recognized by various previous authors. An attempt to provide dynamic alteration of the number of guard channels is discussed in a 1996 IEEE/VTC Proceedings article entitled "Self-Tuning Prioritized Call Handling Mechanism with Dynamic Guard Channels for Mobile Cellular Systems" by Oliver T. W. Yu and Victor C. M. Leung, page 1520. This system utilizes measured current traffic information obtained from adjacent cells to estimate the amount of handoff traffic that is likely to occur in the present cell and the number of guard channels is adjusted in accordance with an algorithm given and explained in the article. This system has an attendant disadvantage of control signaling overhead to obtain the required adjacent traffic information.